Recently, with exhaustion of fossil fuels and concern about environmental pollution, there is an increasing interest in electric vehicles or hybrid vehicles using electrical energy, but not using fossil energy, and in this context, studies are actively being conducted thereon. To supply power required to operate a driving motor for driving of vehicles and actuate various electric devices, a battery pack including a plurality of secondary battery cells is mounted in electric vehicles or hybrid vehicles. The plurality of secondary battery cells included in the battery pack need to uniformly maintain the voltage of each secondary battery cell to ensure stability, a long life span, and a high output.
As a method for uniformly balancing the charging voltage of each secondary battery cell included in a battery pack, many methods are being used, for example, a method that increases the voltage of a secondary battery cell having a relatively lower voltage by supplying a charging current to the corresponding secondary battery cell, a method that decreases the voltage of a secondary battery cell having a relatively higher voltage by discharging the corresponding secondary battery cell, a method that sets a target balancing voltage from the voltage of each secondary battery cell, discharges a secondary battery cell having a voltage higher than the target voltage, and charges a secondary battery cell having a voltage lower than the target voltage, and the like.
These cell balancing methods are implemented by a cell balancing circuit connected to each secondary battery cell. The cell balancing circuit includes a switching element for controlling the start and end of a cell balancing operation, and a discharge resistor used when discharging the secondary battery cell voltage.
However, when an abnormal situation occurs while a cell balancing operation is performed using a cell balancing circuit, for example, when overcurrent instantaneously flows in the cell balancing circuit, or when overvoltage higher than an operating voltage is applied to a switching element, or when overheat is generated through a discharge resistor, a component included in the cell balancing circuit is short-circuited or open-circuited, and consequently, the circuit does not operate normally.
When the cell balancing circuit fails to operate normally, voltage of a secondary battery cell connected to the corresponding circuit excessively increases or decreases in comparison to the other secondary batteries, and as a result, severe results may be produced, for example, operation of a load connected to a battery pack may suddenly stop, or in more serious cases, the battery pack may explode.
To overcome this drawback, there is a need for a diagnosis circuit connected to a cell balancing circuit to diagnose an abnormality in the cell balancing circuit. For example, Japanese Patent Publication No. 2007-085847 discloses an apparatus for detecting an abnormality in a cell balancing circuit, in which a cell balancing circuit including a field effect transistor (FET) and a discharge resistor and a resistor interposed between a source and a drain of the FET are installed at each battery cell, a voltage difference between the source and the drain is measured through the resistor using two comparators to which different levels of reference voltage sources are applied, and whether there is an abnormality in the cell balancing circuit is determined based on a combination of the measured voltage levels (high, low).
However, the above related art needs a separate circuit configuration, that is, the diagnosis circuit to detect an abnormality in the cell balancing circuit, and uses the additional two comparators for each diagnosis circuit, and thus, has a disadvantage of increased manufacturing cost of the apparatus for detecting an abnormality in a cell balancing circuit. Also, the related art can determine whether an abnormality is present in a cell balancing circuit corresponding to each cell, but when an abnormality occurs in all the cell balancing circuits included in the entire cell balancing circuit due to a short or disconnection, the circuit configuration of the related art is inadequate to identify a correct cause of the abnormality in the cell balancing circuit.